Method and apparatus for manufacturing light metal alloy

ABSTRACT

An injection molding system for a metal alloy includes a feeder in which the metal alloy is melted and a barrel in which the liquid metal alloy is converted into a thixotropic state. An accumulation chamber draws in the metal alloy in the thixotropic state through a valve disposed in an opening between the barrel and the accumulation chamber. The valve selectively opens and closes the opening in response to a pressure differential between the accumulation chamber and the barrel. After the metal alloy in the thixotropic state is drawn in, it is injected through an exit port provided on the accumulation chamber. The exit port has a variable heating device disposed around it. This heating device cycles the temperature near the exit port between an upper limit and a lower limit. The temperature is cycled to an upper limit when the metal alloy in the thixotropic state is injected and to a lower limit when the metal alloy in the thixotropic state is drawn into the accumulation chamber from the barrel.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a method and apparatus for manufacturingmetal alloys, more particularly to a method and apparatus formanufacturing a light metal alloy by the process of injection moldingthe metal alloy when it is in a thixotropic (semi-solid) state.

[0003] 2. Description of the Related Art

[0004] One conventional method used to produce molds of metal alloys isthe die cast method. The die cast method is disclosed in U.S. Pat. Nos.3,902,544 and 3,936,298, both of which are incorporated by referenceherein. The die cast method uses liquid metal alloys during casting andas a consequence, metal alloys produced from this method have lowdensities. Metal alloys having low densities are not desirable becauseof their lower mechanical strength, higher porosity, and larger microshrinkage. It is thus difficult to accurately dimension molded metalalloys, and once dimensioned, to maintain their shapes. Moreover, metalalloys produced from die casting have difficulty in reducing theresilient stresses developed therein.

[0005] The thixotropic method improves upon the die casting method byinjection molding a metal alloy from its thixotropic (semi-solid) staterather than die casting it from its liquid state. The result is a metalalloy which has a higher density than one produced from the die castingmethod.

[0006] A method and apparatus for manufacturing a metal alloy from itsthixotropic state is disclosed in U.S. Pat. No. 5,040,589, which isincorporated by reference herein. A method of converting a metal alloyinto a thixotropic state by controlled heating is disclosed in U.S. Pat.Nos. 4,694,881 and 4,694,882, both of which are incorporated byreference herein.

[0007] The system disclosed in U.S. Pat. No. 5,040,589 is an in-linesystem; in which the conversion of the metal alloy into a thixotropicstate and the pressurizing of the same for the purposes of injectionmolding is carried out within a single cylindrical housing. With such asystem, it is difficult to control the molding conditions, i.e.,temperature, pressure, time, etc., and as a result, metal alloys ofinconsistent characteristics are produced.

[0008] Moreover, the system of U.S. Pat. No. 5,040,589 requires that themetal alloy supplied to the feeder be in pellet form. As a consequence,if a mold of undesired characteristics are produced by its system,recycling of the defective molds is not possible unless the defectivemolds are recast in pellet form.

[0009] An improved system for manufacturing light alloy metals, which iscapable of accurately producing molded metal alloys of specifieddimensions within a narrow density tolerance, is desired. Further, aproduction process for light alloy metals which can consistently producemolded metal alloys of desired characteristics, and which can easilyaccommodate recycling of defective molds would represent a substantialadvance in this art.

SUMMARY OF THE INVENTION

[0010] An object of the invention is to provide a method and apparatusfor producing metal alloys through injection molding.

[0011] Another object of the invention is to provide an improvedinjection molding system for metal alloys which is capable of producingmolded metal alloys of accurate dimensions within a narrow densitytolerance.

[0012] Still another object of the invention is to provide an injectionmolding system for light alloy metals which is capable of producinglight alloy metals of desired characteristics in a consistent manner.

[0013] Still another object of the invention is to provide an injectionmolding system for light alloy metals which accommodates recycling ofdefective molds easily.

[0014] These and other objects are accomplished by an improved injectionmolding system for metal alloys in which the steps of melting the metalalloy, converting the metal alloy into a thixotropic state, andinjecting the metal alloy in the thixotropic state into a mold arecarried out at physically separate locations.

[0015] The improved system comprises a feeder in which the metal alloyis melted and a barrel in which the liquid metal alloy is converted intoa thixotropic state. An accumulation chamber draws in the metal alloy inthe thixotropic state through a valve disposed in an opening between thebarrel and the accumulation chamber. The valve selectively opens andcloses the opening in response to a pressure differential between theaccumulation chamber and the barrel.

[0016] After the metal alloy in the thixotropic state is drawn in, it isinjected through an exit port provided on the accumulation chamber. Theexit port has a variable heating device disposed around it. This heatingdevice cycles the temperature near the exit port between an upper limitand a lower limit. The temperature is cycled to an upper limit when themetal alloy in the thixotropic state is injected and to a lower limitwhen the metal alloy in the thixotropic state is drawn into theaccumulation chamber from the barrel.

[0017] A piston-cylinder assembly supplies the accumulation chamber withthe pressure necessary to inject the metal alloy in the thixotropicstate and with the suction necessary to draw in the metal alloy in thethixotropic state from the barrel.

[0018] Additional objects and advantages of the invention will be setforth in the description which follows. The objects and advantages ofthe invention may be realized and obtained by means of instrumentalitiesand combinations particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The invention is described in detail herein with reference to thedrawings in which:

[0020]FIG. 1 is a schematic illustration of a side view of the injectionmolding system according to a first embodiment of the invention;

[0021]FIGS. 2A and 2B illustrates the two positions of a ball valve usedin the injection molding system of the invention;

[0022]FIG. 3 is a schematic illustration of a top view of the injectionmolding system according to a second embodiment of the invention;

[0023]FIG. 4 is a block diagram of an exemplary control circuit for theheating elements of the injection molding system according to theinvention; and

[0024]FIG. 5 shows characteristic curves, corresponding to threesolid/liquid ratios, achievable by the control circuit of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] In the discussion of the preferred embodiment which follows, ametal alloy is produced by injection molding from a magnesium (Mg) alloyingot. The invention is not limited to a Mg alloy and is equallyapplicable to other types of metal alloys. Further, specific temperatureand temperature ranges cited in the description of the preferredembodiment are applicable only to a system producing a Mg alloy, butcould readily be modified in accordance with the principles of theinvention by those skilled in the art in order to accommodate otheralloys. For example, a Zinc alloy becomes thixotropic at about 380°C.-420° C.

[0026]FIG. 1 illustrates an injection molding system 10 according to afirst embodiment of the invention. The system 10 has four substantiallycylindrical sections—a feeder 20, a barrel 30, a cylinder 40, and anaccumulation chamber 50. A metal alloy, e.g., Mg alloy, is supplied tothe feeder 20. The feeder 20 is provided with a mixer 22 and a heatingelement 25 disposed around its outer periphery. The heating element 25may be of any conventional type and operates to maintain the feeder 20at a temperature high enough to keep the metal alloy supplied throughthe feeder 20 in a liquid state. For a Mg ingot, this temperature wouldbe about 600° C. or greater. The mixer 22 is driven by a stirrer motor23 for the purposes of evenly distributing the heat from the heatingelement 25 to the metal alloy supplied to the feeder 20.

[0027] The liquid metal alloy is subsequently supplied to the barrel 30by way of gravity through an opening 27 which may optionally be suppliedwith a valve serving as a stopper (not shown). The barrel 30 has aplurality of heating elements 70 a-e are disposed along the length ofthe barrel 30. The heating elements 70 a-e are maintain the barrel attemperatures at and slightly below the melting point of the liquid metalalloy supplied from the feeder 20. For an injection molding system 10designed for a Mg ingot, heating pairs 70 a and 70 b would be maintainedat a temperature of about 600° C.; a heating pair 70 c would bemaintained at a temperature of about 580° C.; and heating pairs 70 d and70 e would be maintained at a temperature of about 550° C. Heating pairs70 a- 70 e induce a thermal slope to the metal alloy flowing through thebarrel 30. The purpose of the thermal slope is to convert liquid metalalloy entering the barrel 30 into a metal alloy in the thixotropic stateat the exit of the barrel 30.

[0028] The barrel 30 also has a physical slope or an inclination. Theinclination, preferably between 30° and 90°, is necessary to supply themetal alloy in the thixotropic state to the accumulation chamber 50 bythe force of gravity. The barrel 30 is also provided with a mixer 32which is driven by a stirrer motor 33. The mixer 32 is provided toassure that the ratio of solid and liquid is consistent throughout themetal alloy in the thixotropic state. Plural mixing blades attached tothe rotating shaft may of course be used.

[0029] The metal alloy in the thixotropic state exits the barrel 30 intoan accumulation chamber 50 through a ball valve 60. The ball valve 60operates in response to a pressure differential between the accumulationchamber 50 and the barrel 30. The pressure within the barrel 30 remainssomewhat constant, but the pressure within the accumulation chamber 50is determined by the position of a piston 45 disposed in the cylinder40. When the piston 45 is displaced inwardly, the pressure in theaccumulation chamber 50 increases (and becomes higher than that of thebarrel 30) and the ball valve 60 closes off an opening 37 between thebarrel 30 and the accumulation chamber 50. When the piston 45 isdisplaced outwardly, the pressure in the accumulation chamber 50decreases and is lower than that of the barrel 30, and the ball valve 60opens. A seal 41, e.g., an O-ring, is provided at the outer periphery ofthe piston 45 to maintain the pressure within the accumulation chamber50 and to prevent leakage of metal alloy in the thixotropic state drawninto the accumulation chamber 50.

[0030] The operation of the ball valve 60 is shown in greater detail inFIGS. 2A and 2B. FIG. 2A shows the position of the ball valve 60 whenthe piston 45 is displaced outwardly. In this case, the opening 37between the barrel 30 and the accumulation chamber 50 is opened as theball element 65 of the ball valve 60 moves away from the opening 37. Aball valve stop 62 is provided to confine the ball valve movement awayfrom the opening 37. On the other hand, when the piston 45 is displacedinwardly, as shown in FIG. 2B, the pressure inside the accumulationchamber 50 increases and the ball element 65 of the ball valve 60 isforced to lodge up against the opening 37 and thereby close off fluidcommunication between the barrel 30 and the accumulation chamber 50.

[0031] In a slightly different embodiment, the ball valve 60 may beprovided with a biasing element, e.g., a spring. In such a case, theball element 65 may be biased towards either the open or the closedposition. It is preferable to provide such a biasing element in largerinjection molding systems for producing metal alloys.

[0032] In still another slightly different embodiment, the ball valve 60may be electronically controlled, in which the opening and closing ofthe ball valve would be synchronized with the displacement motion of thepiston 45.

[0033] As shown in FIG. 1, heating elements 70 f- 70 i and heatingelement 80 are also provided along the lengths of the cylinder 40 andthe accumulation chamber 50. Heating elements referenced and prefixed bythe numeral 70 are resistance heating elements. In the preferredembodiment of the injection molding system for producing a Mg alloy,heating pairs 70 f-70 i are preferably maintained at temperatures of550-570° C. in order to maintain the metal alloy in a semi-solid state.

[0034] The heating element 80 is an induction coil heater and is used tocycle the temperature at an exit port 57 of the accumulation chamber 50between temperatures 550° C. and 580° C. One cycle is approximately 30seconds to one minute. As the temperature at the exit port 57 is cycled,the characteristic of the metal alloy in the thixotropic state near theexit port 57 is varied. For example, the exit port 57 at a temperatureof 550° C. would cause the metal alloy in the thixotropic state to havea higher solid to liquid ratio compared with the situation in which theexit port 57 is at a temperature of 580° C.

[0035] The purpose of raising the solid to liquid ratio of the metalalloy in the thixotropic state at the exit port 57 during the outwardstroke of the piston 45 is to solidify the metal alloy in thethixotropic state near the exit port 57 sufficiently to function as aplug for the accumulation chamber 50. During the inward stroke of piston45, the temperature at the exit port 57 cycled to a higher temperature(e.g., 580° C.) so that the metal alloy in the thixotropic state at theexit port 57 will take on a characteristic with a lower solid/liquidratio and thereby allow the metal alloy in the thixotropic state to beeasily injected through the exit port 57.

[0036] The injection of the metal alloy in the thixotropic state is madethrough the exit port 57 into a mold (not shown). Molds of desiredcharacteristics are retained and molds of undesired characteristics arerecycled to the feeder 20. The defective molds (e.g., density of moldoutside a predetermined range, surface blemish, etc.) are recycled “asis” and need not be reformed into any particular shape, since the systemaccording to the invention melts the metal alloy supplied thereto beforefurther processing.

[0037] The control of the heating elements 70, the cycling of theinduction coil heating element 80, and the timing of the piston strokeare implemented electronically based on the following. The heatingelements 70 are resistance heating elements. Electric current issupplied through the heating elements 70 sufficiently to maintain theheating elements 70 at their desired temperatures. The cycling of theinduction coil heating element 80 is synchronized with the pistonstroke. An outward piston stroke should be synchronized with the lowertemperature and an inward piston stroke should be synchronized with theupper temperature. The control of the piston stroke is accomplished in aconventional manner.

[0038] The following table gives representative dimensions for a large,medium and small injection molding systems for metal alloys. SystemBarrel Cylinder Chamber Port Size 30 40 50 57 Large d:60 d:52 d:52 d:12l:120 l:1500 l:1500 Medium d:50 d:36 d:36 d:10 l:110 l:700 l:700 Smalld:40 d:32 d:32 d:10 l:100 l:700 l:700

[0039] The dimensions given in the above table are exemplary and areprovided to give guidance on how scaling for large, medium and smallsystems should be carried out. In the table, d indicates the insidediameter and l indicates the length. All dimensions are in millimeters(mm).

[0040]FIG. 3 is a top view illustration of a second embodiment of theinjection molding system of the present invention. This embodiment isidentical to the first embodiment except for the barrel 30. The barrel30 in FIG. 3 is positioned horizontally with respect to the cylinder 40and the accumulations chamber 50. Since gravity no longer supplies theforce necessary to advance the metal alloy in the thixotropic stateflowing in the barrel 30, a plurality of screw elements 34 driven by themotor 33 is provided. The screw elements 34 advance the metal alloy inthe thixotropic state to accumulate near the opening 37 adjacent to theball valve 60. The mixer 32 is provided on the same shaft 35 whichrotates the screw elements 34. (In FIG. 3, the shaft 35 is shown to beseparated by the feeder 20, because the shaft 35 runs underneath thefeeder 20.) Therefore, the motor 33 operates to power both the screwelements 34 and the mixer 32. Other features of this embodiment areidentical to the first embodiment.

[0041] Both the first and second embodiments may also have a pressuredevice attached to the barrel 30 to slightly pressurize the barrel. Suchpressure is much less than the pressure used in the cylinder 40 and theaccumulation chamber 50.

[0042] In all of the embodiments of the invention it is desired to havea temperature gradient between the portion of the barrel 30 in which themetal alloy enters the barrel 30 and the portion of the opening 37 wherethe metal alloy in the thixotropic state exits the barrel 30. Thetemperature gradient is necessary in order to produce the metal alloy inthe thixotropic state. An exemplary manner of producing the temperaturegradient is shown in FIGS. 4 and 5. As seen in FIG. 4, thecontrol-apparatus includes a control device 100 and a power supplycircuit 102. The power supply circuit is connected to each of theheating element pairs 70 a- 70 i and supplies different currents for theresistive heaters. Thus, a larger current (or a current supplied for alonger time, or a combination of current value and time) supplied fromthe power supply to a particular heating element or pair, say pair 70 a,results in a larger heating effect in the resistive heater pair.

[0043] Each of the heating pairs 70 a- 70 e heats a respective localizedzone in the barrel 30. By controlling the current (and/or time) suppliedto the heating pairs 70 a- 70 e, the amount of heat in each zone of thebarrel 30 adjacent the respective heating pair may be controlled. Whileonly five heating pairs 70 a- 70 e are shown provided for the barrel 30,the barrel 30 is preferably equipped with between seven to tenseparately controllable heating zones, each corresponding to aseparately controllable heating pair.

[0044] Preferably, the control device is programmable so that thedesired solid/liquid ratio characteristic R1, R2, R3 of the metal alloyin the thixotropic state may be achieved as seen in FIG. 5. Controldevice 100 may, for example, comprise a microprocessor (with anassociated input device such as a keyboard, not shown) which may beeasily and quickly reprogrammed to changed the resultant solid/liquidratio depending on the type of finished mold product desired. FIG. 5shows three characteristic curves for three different values, R1, R2,and R3 of the solid/liquid ratio. The abscissa of the graph in FIG. 5 islabeled “a, b, . . . e” corresponding to the position of the respectiveheating pairs 70 a, 70 b . . . 70 e in FIGS. 1 and 3. The ordinate ofFIG. 5 represents the varying temperature range which may be employed.It should be appreciated that all values of the temperature used for theheating pairs 70 a, 70 b . . . 70 e are within the range of 550° C. to580° C. necessary to maintain the metal alloy in its thixotropic state.Further, it will be noted that the values of the temperature associatedwith the position of heating pair 70 a are approximately the same (580°C.) for all the curves since these values are near the value of themetal alloy as it enter the barrel 30 from the feeder 20. By selecting aratio R1, as contrasted with R3, one may achieve a larger solid/liquidratio and thus achieve a more dense resultant metal alloy in thethixotropic state and a more dense molded product. The heating elementpairs 70 f-70 i are all typically controlled to have a temperature equalto the temperature of the heating pair 70 e, i.e., there is notemperature gradient between heating pairs 70 f-70 i.

[0045]FIG. 4 also shows the use of position detecting devices used withan electrically actuated valve 104 which may be used instead of the ballvalve 60. The electrically actuated valve 104 has two positions, onepermitting communication between the barrel 30 and accumulation chamber50 and the other blocking such communication. The valve is controlled bythe power supply circuit as shown by the dotted line 106. Two limitswitches S1 and S2 are used to open and close valve 104. These limitswitches are shown implemented in the form of two photodetectors 108 and110 and associated light sources 112 and 114 (i.e., photodiodes).Detector 108 provides an output signal along line 116 to the controldevice 100 whenever the light beam from the source 112 is interrupted bythe piston 45 moving outwardly (to the right in FIGS. 1 and 3) and thusacts as a first switch S1. In response to this signal the control valve104 is opened permitting the metal alloy in the thixotropic state toenter the accumulation chamber 50 from the barrel 30. Also, this samesignal may be used to direct the power supply circuit to cool down theinduction coil heating element 80 to a relatively low temperature (550°C.) thus permitting the solid/liquid ratio of the metal alloy in thethixotropic state which is adjacent the exit port 57 to increase andthus form a plug.

[0046] When the piston 45 reaches its outermost position as shown by thedotted lines 45′ in FIGS. 1 and 3, the second limit switch (light source114 and photodetector 110) is actuated for delivering a signal alongline 118 to the control device 100 thus acting as a second switch S2(e.g., see FIG. 4). In response to this signal, the control device 100directs the power supply circuit 102 to close valve 104 and to raise thetemperature of the induction coil heating element 80 to thereby lowerthe solid/liquid ratio of the metal alloy in the thixotropic state inthe region of the exit port 57 and unplug the exit port 57 to permitinjection to take place upon the inward movement of the piston 45.

[0047] In the above described manner, the gradient temperature may beselectively controlled, and the induction coil heating element 80 may becontrolled in synchronism with the movement of the piston 45. Moreover,in the case of an electronically actuated valve, the valve opening andclosing may also be controlled in synchronism with the movement of thepiston 45.

[0048] While particular embodiments according to the invention have beenillustrated and described above, it will be clear that the invention cantake a variety of forms and embodiments within the scope of the appendedclaims. For example, the photodetectors and light sources may bereplaced by mechanical micro-switches, or the position of the piston 45may be inferred by measuring pressure changes within the accumulationchamber 50. Alternatively, an encoder (e.g. photo-encoder) may be usedto detect the position of the shaft 45.

What is claimed is:
 1. A method of injection molding a metal alloycomprising the steps of: (a) drawing into a chamber said metal alloy ina thixotropic state; and (b) injecting said metal alloy in thethixotropic state from said chamber into a mold.
 2. A method ofinjection molding a metal alloy as recited in claim 1 , furthercomprising the step of: (c) cycling the temperature of a heating devicedisposed near a port in said chamber through which said metal alloy inthe thixotropic state is injected, said cycling being synchronized withsteps (a) and (b).
 3. A method of injection molding a metal alloy asrecited in claim 2 , wherein during step (a), the temperature of theheating device is cycled to a lower value and during step (b), thetemperature of the heating device is cycled to an upper value.
 4. Amethod of injection molding a metal alloy as recited in claim 1 ,further comprising, before step (a), the steps of: supplying and meltingthe metal alloy into a liquid state; and cooling the metal alloy in theliquid state into the thixotropic state.
 5. An injection molding systemfor producing a metal alloy, comprising: an accumulation chamber whichstores therein the metal alloy in a thixotropic state, said chamberhaving an exit port through which the metal alloy in the thixotropicstate is injected; a variable heating device disposed near the exitport, said heating device cycling the temperature near the exit portbetween an upper value and a lower value, the temperature near the exitport being cycled to the upper value when the metal alloy in thethixotropic state is injected.
 6. An injection molding system forproducing a metal alloy as recited in claim 5 , wherein said heatingdevice is an induction heating coil.
 7. An injection molding system forproducing a metal alloy as recited in claim 6 , further comprising apiston-cylinder assembly which supplies said accumulation chamber withpressure for injecting the metal alloy in the thixotropic state.
 8. Aninjection molding system for producing a metal alloy as recited in claim7 , further comprising:. a barrel which feeds said accumulation chamberwith the metal alloy in the thixotropic state; and a valve disposed inan opening between said barrel and said accumulation chamber, said valveselectively opening and closing said opening in response to a operationof said piston-cylinder assembly.
 9. An injection molding system forproducing a metal alloy as recited in claim 5 , further comprising apiston-cylinder assembly which supplies said accumulation chamber withpressure for injecting the metal alloy in the thixotropic state.
 10. Aninjection molding system for producing a metal alloy as recited in claim9 , further comprising: a barrel which feeds said accumulation chamberwith the metal alloy in the thixotropic state; and a valve disposed inan opening between said barrel and said accumulation chamber, said valveselectively opening and closing said opening in response to a operationof said piston-cylinder assembly.
 11. An injection molding system forproducing a metal alloy as recited in claim 10 , wherein saidpiston-cylinder assembly comprises a piston and a cylinder, whereinmovement of said piston outwardly from said cylinder draws said metalalloy in the thixotropic state into said accumulation chamber from saidbarrel, and movement of said piston inwardly into said cylinder injectssaid metal alloy in the thixotropic state from said accumulation chamberinto a mold.
 12. An injection molding system for producing a metal alloyas recited in claim 11 , wherein said valve is electronically controlledand said system further comprises means for detecting the position ofsaid piston and for controlling said electronically controlled valve inresponse thereto.
 13. An injection molding system for producing a metalalloy as recited in claim 12 further comprising means for controllingsaid variable heating device in response to said detector means.
 14. Aninjection molding system for producing a metal alloy as recited in claim13 wherein said variable heating device is controlled to cycle to saidupper value when said detecting means detects a first predeterminedposition of said piston corresponding to said piston extending maximallyfrom said cylinder to thereby permit injection of said metal alloy insaid thixotropic state, and is controlled to cycle to said lower valuewhen said detecting means detects a second predetermined position ofsaid piston corresponding to said piston extending minimally from saidcylinder to thereby permit said metal alloy in said thixotropic state toform a plug in said exit port of said accumulation chamber.
 15. Aninjection molding system for producing a metal alloy as recited in claim11 further comprising: means for detecting the position of said piston;and means for controlling said variable heating device in response tosaid detector means.
 16. An injection molding system for producing ametal alloy as recited in claim 15 wherein said variable heating deviceis controlled to cycle to said upper value when said detecting meansdetects a first predetermined position of said piston corresponding tosaid piston extending maximally from said cylinder to thereby permitinjection of said metal alloy in said thixotropic state, and iscontrolled to cycle to said lower value when said detecting meansdetects a second predetermined position of said piston corresponding tosaid piston extending minimally from said cylinder to thereby permitsaid metal alloy in said thixotropic state to form a plug in said exitport of said accumulation chamber.
 17. An injection molding system forproducing a metal alloy, comprising: an accumulation chamber whichstores therein the metal alloy in a thixotropic state, said chamberhaving an exit port through which the metal alloy in the thixotropicstate is injected; a barrel which feeds said accumulation chamber withthe metal alloy in the thixotropic state; and a valve disposed in anopening between said barrel and said accumulation chamber, said valveselectively opening and closing said opening in response to a pressuredifferential between said accumulation chamber and said barrel.
 18. Aninjection molding system for producing a metal alloy as recited in claim17 , further comprising a piston-cylinder assembly which supplies saidaccumulation chamber with pressure for injecting the metal alloy in thethixotropic state.
 19. An injection molding system for producing a metalalloy as recited in claim 17 , wherein said valve is a ball valve. 20.An injection molding system for producing a metal alloy, comprising: anaccumulation chamber which stores therein the metal alloy in athixotropic state, said chamber having an exit port through which themetal alloy in the thixotropic state is injected; a barrel which feedssaid accumulation chamber with the metal alloy in the thixotropic state,said barrel positioned to gravity feed said metal alloy to saidaccumulation chamber; a piston-cylinder assembly having a piston and acylinder wherein movement of said piston outwardly from said cylinderdraws said metal alloy in the thixotropic state into said accumulationchamber from said barrel, and movement of said piston inwardly into saidcylinder injects said metal alloy in the thixotropic state from saidaccumulation chamber into a mold; and a valve disposed in an openingbetween said barrel and said accumulation chamber, said valveselectively opening and closing said opening in response to a one of (a)a pressure differential between said accumulation chamber and saidbarrel caused by movement of said piston, and (b) movement of saidpiston.
 21. An injection molding system for producing a metal alloy asrecited in claim 20 wherein said barrel is positioned at an angle ofbetween 30 and 90 degrees relative a horizontal direction, and saidaccumulation chamber has a longitudinal axis oriented in a horizontaldirection.
 22. An injection molding system for producing a metal alloy,comprising: an accumulation chamber which stores therein the metal alloyin a thixotropic state, said chamber having an exit port through whichthe metal alloy in the thixotropic state is injected; and a barrel whichfeeds said accumulation chamber with the metal alloy in the thixotropicstate, said barrel positioned to gravity feed said metal alloy to saidaccumulation chamber.
 23. An injection molding system for producing ametal alloy as recited in claim 22 wherein said barrel is positioned atan angle of between 30 and 90 degrees relative a horizontal direction,and said accumulation chamber has a longitudinal axis oriented in ahorizontal direction.
 24. A method of injection molding a metal alloycomprising the steps of: (a) producing said metal alloy in a thixotropicstate in a first chamber; (b) gravity feeding said metal alloy in thethixotropic state from said first chamber to an injection chamber; and(c) injecting said metal alloy in the thixotropic state from saidinjection chamber into a mold.
 25. A method of injection molding a metalalloy as recited in claim 24 , further comprising the steps of:supplying said metal alloy into a feeder and melting said metal alloytherein prior to said step (a); and supplying the melted metal alloy tosaid first chamber.
 26. A method of injection molding a metal alloy asrecited in claim 25 , further comprising the step of: (d) recycling adefective mold by supplying the defective mold into the feeder.